Over the past fifteen to twenty years, micro-mirror based spatial light modulators (SLM) have made incremental technical progress and have gained acceptance in the display industry. The devices operate by tilting individual micro-mirror plates in the array around a torsion hinge with an electrostatic torque to deflect the incident light in a predetermined exit direction. In a more popular digital mode operation, the directional light is turned “on” or “off” by selectively rotating the individual mirrors in a micro-mirror array and mechanically stopped at a specific landing position to ensure the precision of deflection angles. A functional micro-mirror array requires low contact sticking forces at the mechanical stops and high efficiency electrostatic torques to control timing, to overcome surfaces contact adhesion, and to ensure reliability. A high performance spatial light modulator for display applications produces high brightness and high contrast ratio videos images.
Early SLM in video applications suffers a disadvantage of low brightness and low contrast ratio of the projected images. Previous SLM designs typically have a low active reflection area fill-ratio of pixels (e.g., ratio between active reflective areas and non-active areas in each pixel). A large inactive area around each pixel in the array of SLM results in a low optical coupling efficiency and low brightness. The scattered light from these inactive areas in the array forms diffraction patterns that adversely impact the contrast of video images. Another factor reducing the contrast ratio of micro-mirror array based SLM is the diffraction of the scattered light from two straight edges of each mirror in the array that are perpendicular to the incident illumination. In a traditional square shape mirror design, orthogonal incident light is scattered directly by the perpendicular straight leading and trailing edges of each mirror in the array during the operation. The scattered light produces a diffraction pattern and the projection lenses collect much of the diffracted light. The bright diffraction pattern smears out the high contrast of projected video images.
One type of micro-mirror based SLM is the Digital Mirror Device (DMD), developed by Texas Instruments. The most recent implementations include a micro-mirror plate suspended via a rigid vertical support post on top of a yoke plate. The yoke plate further comprises a pair of torsion hinges and two pairs of horizontal landing tips above addressing electrodes. The electrostatic forces on the yoke plate and mirror plate controlled by the voltage potentials on the addressing electrodes cause the bi-directional rotation of both plates. The double plate structure is used to provide an approximately flat mirror surface that covers the underlying circuitry and hinge mechanism, which is one way to achieve an acceptable contrast ratio.
However, the vertical mirror support post that elevates the mirror plate above the hinge yoke plate has two negative influences on the contrast ratio of the DMD. First, a large dimple (caused by the fabrication of mirror support post) is present at the center of the mirror in current designs that causes scattering of the incident light and reduces optical efficiency. Second, double plate rotation causes a horizontal displacement of mirror surfaces along the surface of DMD, resulting in a horizontal vibration of a micro-mirror during operation. The horizontal movement of mirrors requires extra larger gaps to be designed in between the mirrors in the array, reducing the active reflection area fill-ratio further. For example, if the rotation of the mirror in each direction is 12°, every one micron apart between the mirror and the yoke results in 0.2 microns horizontal displacement in each direction. In other words, more than 0.4 microns spacing between the adjacent mirrors is required for every one micron length of mirror support post to accommodate the horizontal displacement.
The yoke structure limits the electrostatic efficiency of the capacitive coupling between the bottom electrodes and the yoke and mirror. Especially in a landing position, the yoke structure requires a high voltage potential bias between the electrodes and the yoke and mirror to enable the angular crossover transition. Double plate structures scatter incident light which also reduces the contrast ratio of the video images.
Another prior art reflective SLM includes an upper optically transmissive substrate held above a lower substrate containing addressing circuitry. Two hinge posts from the upper substrate suspend one or more electrostatically deflectable elements. In operation, individual mirrors are selectively deflected and serve to spatially modulate light that is incident to, and then reflected back through, the upper transmissive substrate. Motion stops may be attached to the reflective deflectable elements so that the mirror does not snap to the bottom control substrate. Instead, the motion stop rests against the upper transmissive substrate thus limiting the deflection angle of the reflective deflectable elements.
In such a top hanging mirror design, the mirror hanging posts and mechanical stops are all exposed to the light of illumination, which reduces the active reflection area fill-ratio and optical efficiency, and increases the light scattering. It is also difficult to control the smoothness of reflective mirror surfaces, which is sandwiched between the deposited aluminum film and LPCVD silicon nitride layers. Deposition film quality determines the roughness of reflective aluminum surfaces. No post-polishing can be done to correct the mirror roughness.
In would be highly desirable to provide a high contrast spatial light modulator that overcomes the foregoing shortcomings associated with prior art techniques.